Fracture analysis of double-layer hard and thick roof and the controlling effect on strata behavior: A case study

The movement of double-layer hard and thick main roofs during underground coal extraction via longwall top coal caving (LTCC) differs from that of other types of overlying strata. Therefore, numerous problems such as roadway instability, rockburst, and hydraulic cylinder damage will be encountered as a result of the instantaneous fracture of a double-layer hard and thick main roof. The key to controlling the behavior of strata is to understand the movement and fracture pattern of the double-layer hard and thick roof. The Xinhe mine was chosen for the case study because it is typical of the type of mine in which the LTCC method is adopted, and the minable coal seam usually underlies a double-layer, hard, and thick sandstone. The mining-induced fracturing and movement of the roof were investigated via microseismic (MS) monitoring. The structural characteristics affected by roof fracturing were determined, which helped in explaining the strong strata behavior. In addition, in order to control the local damage caused by the instantaneous fracture of the roof, a suitable long-hole pre-split blasting technique was adopted to weaken the strength and massiveness of the double-layer hard and thick roof. The total length of the longwall panel could be smoothly extracted without any further roadway instability after the systematic adoption of the proposed long-hole pre-split blasting technique.